The present invention relates to a method for determining and varying the thermal characteristics of a liquid cooling system and more particularly to a method for determining the thermal characteristics of a water cooling system having a surface exposed to the atmosphere and modifying the flow pattern of the system in response to the determined thermal characteristics.
In thermal electric power generating plants and in other manufacturing processes, water or other liquid cooling media are a necessary part of proper plant operation. Natural fresh and saline water supplies are particularly attractive from the standpoint of economy and are thus ordinarily utilized for cooling purposes and in most large scale operations in which cooling is necessary. For example, large electric power generating plants require a liquid cooling media to condense the stream which drives the turbines and are thus almost exclusively situated adjacent natural water sources such as lakes and rivers so that water from the natural source may be utilized either directly for cooling purposes or as a source of make-up water for closed loop cooling reservoirs.
In the cooling system where the water is used directly, i.e. a once-through system, water from the natural reservoir is routed through the plant cooling system and is thereafter discharged into the natural reservoir. A closed loop or recirculating cooling system, on the other hand, involves the use of water from a man-made reservoir and the cooling water is circulated in a closed loop through the reservoir and the plant cooling system. In either case, the cooling water absorbs heat as it passes through the plant cooling system and the heated water is then discharged into the cooling water reservoir.
Ideally, in a closed loop, recirculating system, the cooling water should give up at least the same amount of heat which it absorbs before it is reused for cooling purposes. If not, the efficiency of the plant may suffer during periods in which maximum plant output is required. If the heated cooling water is to be discharged into a natural reservoir as in a once-through cooling system, it may be mandatory for the temperature of the discharged water to fall below a certain prescribed maximum limit before being discharged into the natural reservoir.
The thermal characteristics of any cooling system of the above-described type having a fixed surface area exposed to the atmosphere may vary immensely with plant loading conditions and prevailing meteorological conditions, both of which may be continuously changing. It may thus be particularly difficult to maximize plant efficiency and/or insure compliance with regulations concerning water discharge temperature using known closed, recirculating types of systems or once-through systems. The answer to this complex cooling problem has often necessarily been a costly over-design of the cooling system or merely tolerance of periods of low efficiency plant operation or periods of noncompliance with water discharge temperature standards.
Of course, an optimum solution to the problems encountered in known cooling systems may require an accurate awareness of reservoir thermal characteristics and cooling requirements for the prevailing plant load conditions. With this information available during and after the initial reservoir design stages, the types of reservoir cooling system initially selected and the method of operating that particular type of cooling system can be varied to optimize the cooling needs of the plant while maintaining the set standards of water temperature discharge.
It has been suggested that the thermal characteristics of a cooling reservoir may be determined by placing a number of thermometers or other suitable temperature sensors at predetermined locations throughout the cooling system. To employ this approach, either the temperature measured at each isolated location in the cooling system must be assumed to be representative of the overall thermal characteristics of the cooling fluid at and between each location or enough temperature measuring devices must be utilized to provide an accurate indication of the overall thermal characteristics of the cooling system. The assumption required if only a few temperature sensors are used may not be a valid assumption because of the wide temperature variations ordinarily encountered in a cooling reservoir. The use of a large number of temperature sensors may provide sufficient accuracy but may also be extremely expensive and complex. Moreover, this approach may not permit any reasonably accurate predictive studies which are highly useful in both the design of the cooling system and the planning required for its optimum use. Thus, not only may such instrumentation be extremely expensive, it may not provide the flexibility and accuracy required in applications such as power plant cooling reservoirs.
Another approach which may permit a degree of design and operation predictability on a gross or long-term basis involves the calculation of heat transfer across the exposed surface of the body of water on a longterm average basis taking into account the major mechanisms of heat transfer. For example, "The Johns Hopkins University Cooling Water Studies for Edison Electric Institute," research project RP-49 by Dr. John E. Edinger et al, which is available from Edison Electric Institute, 750 3rd Avenue, New York, N.Y., sets forth certain equations for the transfer of heat across a water surface on a gross basis as it relates to cooling reservoirs having a surface exposed to atmosphere. The equations are based upon average climatological conditions over fairly long periods of time and thus provide gross heat exchange values which may be usable in approximating the required characteristics of the cooling system for a particular application.
Of course, the average or gross data obtained using these equations may not be useful in controlling the hour-by-hour operation of a cooling reservoir required to optimize the operation of the power plant and to maintain the cooling water discharge temperatures within the set standards. Moreover, it appears from the above Edinger et al study that a discrepancy may exist between calculated and empirical data even when calculated on an average or gross basis.
It is accordingly an object of the present invention to provide a novel method for determining the thermal characteristics of a cooling reservoir on an essentially real time basis.
It is a further object of the present invention to provide a novel method for determining the thermal characteristics of at least a portion of a cooling system at selected locations in the system whereby the cooling system may be controlled in accordance with the prevailing reservoir thermal characteristics.
It is yet another object of the present invention to provide a novel method for controlling a cooling reservoir to provide the required reservoir cooling capacity for a predetermined power plant heat load.
It is a further object of the present invention to provide a novel method for optimizing the efficiency of a power plant on an essentially real time basis in response to determined thermal characteristics of the power plant cooling system.
It is a more specific object of the present invention to provide a novel method for providing at least hourly accounting of water temperatures throughout at least a portion of a cooling system reservoir of a thermal power generating station.
It is another specific object of the present invention to provide a novel method for determining the natural equilibrium temperature of segments of a cooling system reservoir of a thermal power generating station whereby the temperatures of the segments are available on a relatively short, periodic basis for use in controlling the operation of the cooling system on a real time or predictive basis.
One embodiment of the present invention intended to accomplish at least some of the foregoing objects comprises establishing in a cooling system reservoir a plurality of reservoir segments each representing a specified time interval, preferably equal time intervals, determining an initial thermal characteristic of at least one of the segments at a first time and at a predetermined location in the cooling system, thereafter determining the thermal characteristic of the at least one segment at the specified time intervals, e.g., equal time intervals after the first time, and controlling the cooling system flow pattern in response to the determined thermal characteristic.
In a preferred form of the invention, the thermal characteristic of the segment is determined by calculating the temperature of segment in response to the initially determined characteristic and prevailing meteorological conditions affecting heat transfer across the surface of the segment. An equilibrium temperature is established for the segment and, to the extent that the prevailing meteorological conditions affect the equilibrium temperature, the calculated temperature of the segment is increased or decreased.